JP2018147882A - Solvent for coating positive electrode active material of secondary cell, positive electrode active material slurry containing the same, and secondary cell manufactured therefrom - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solvent for coating positive electrode active material of secondary cell, having a low viscosity regardless of high solid content density, and indicating excellent dispersion stability for the positive electrode active material, and to provide a positive electrode active material slurry containing the same, a positive electrode including a positive electrode active material layer formed therefrom, and a secondary cell including the same.SOLUTION: A solvent for coating positive electrode active material of secondary cell contains N-ethyl formamide. A positive electrode active material slurry contains a positive electrode active material, a binder and N-ethyl formamide. The positive electrode active material of the positive electrode active material slurry is represented by formula 1; Li+x(NiaCobMnc)O(-0.5≤x≤0.6, 0≤a, b, c≤1, x+a+b+c=1). The binder is selected from a fluorine-based binder and a rubber-based binder, and the conductive material is a carbon-based material in the positive electrode active material slurry. A positive electrode contains a positive electrode active material layer formed of these positive electrode active material slurries, and a secondary cell includes the positive electrode.SELECTED DRAWING: None

Description

The present invention relates to a positive electrode active material coating solvent for a secondary battery, a positive electrode active material slurry containing the same, and a secondary battery produced therefrom.

Along with the rapid development of the communication industry such as the electronic industry and various information communications including mobile communications, in order to meet the demands for electronic devices to be lighter, thinner and smaller, notebook computers, netbooks, tablet PCs, mobile phones, smartphones, PDAs, digital Portable electronic products such as cameras and camcorders and communication terminal devices are widely used. Along with this, there is an increasing interest in the development of batteries that are the driving power sources for these devices.

In addition, with the development of electric vehicles such as hydrogen electric vehicles, hybrid vehicles, and fuel cell vehicles, great interest is focused on the development of batteries with high performance, large capacity, high density, high output, and high stability. The development of batteries having fast charge / discharge rate characteristics has also become a major issue.

Along with this trend, lithium secondary batteries having a high energy density and voltage, a long cycle life and a low self-discharge rate have been commercialized and widely used, thereby maximizing the above effects. Therefore, research on lithium secondary batteries has been actively conducted.

The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte as basic components. The positive electrode and the negative electrode are electrodes in which energy conversion and storage such as oxidation and reduction occur, and have positive and negative potentials, respectively. The separator is located between the positive electrode and the negative electrode, maintains electrical insulation, and provides a charge transfer path. In addition, the electrolyte serves as a mediator of charge transfer.

In a lithium secondary battery that is an example of a secondary battery, the performance (for example, capacity) of the battery is greatly influenced by the positive electrode active material used. In order to improve the performance of such a lithium secondary battery, the positive electrode active material must be formed as a layer having a uniform and stable thickness on the current collector while being able to be loaded with an appropriately high numerical value. In order to solve such a problem, it must be easy to adjust the solid content, viscosity, and the like of the positive electrode active material slurry.

At this time, the positive electrode of the lithium secondary battery can be manufactured by applying a positive electrode active material slurry containing a positive electrode active material on a positive electrode current collector and drying it. The positive electrode active material slurry may be a fluid mixture in which a binder and an organic solvent are added to and mixed with the positive electrode active material.

Since a normal positive electrode active material has a small particle size and a large carbon-coated specific surface area, a large amount of an organic solvent has a low solid content concentration (45% solid content level) in the positive electrode active material slurry containing the normal positive electrode active material. The above problem is solved by adding (for example, N-methyl-2-pyrrolidone (NMP) or the like) and adjusting the viscosity appropriately. However, when a large amount of organic solvent is used, an undried portion may be formed even after drying, so that high-loading is impossible and process speed such as drying speed is increased. Triggers the problem of reduced productivity. Further, the positive electrode active material slurry having a low solid content concentration has a problem in that dispersion of the positive electrode active material layer may cause variation in layer thickness and loading due to a decrease in dispersion stability. In particular, there is a problem that it is not environmentally friendly due to the harmfulness of the organic solvent.

Under the background as described above, the present inventors diligently studied to improve the above-mentioned problems. As a result, a slurry containing a positive electrode active material is obtained by using a coating solvent containing N-ethylformamide. A positive electrode having improved capacity characteristics and life characteristics by stably forming a positive electrode active material layer with a uniform thickness, and a secondary battery including the same can be remarkably improved. The present invention has been completed by confirming that it can be provided.

KR10-0436708B1 KR10-1764470B1

An object of the present invention includes a positive electrode active material coating solvent for a secondary battery that has a low viscosity and exhibits excellent dispersion stability with respect to the positive electrode active material despite the high solid content concentration, and the same The object is to provide a positive electrode active material slurry.

Another object of the present invention is to provide a positive electrode including a positive electrode active material layer formed from the positive electrode active material slurry, and a secondary battery including the positive electrode.

In order to solve the above problems, the present invention provides a solvent for coating a positive electrode active material of a secondary battery containing N-ethylformamide.

The present invention also provides a positive electrode active material slurry containing a positive electrode active material, a binder, and N-ethylformamide.

In the positive electrode active material slurry according to an embodiment of the present invention, the positive electrode active material may be represented by the following Chemical Formula 1.

[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂
(In the chemical formula 1, −0.5 ≦ x ≦ 0.6, 0 ≦ a, b, c ≦ 1, and x + a + b + c = 1.)

In the positive electrode active material slurry according to an embodiment of the present invention, the binder may be selected from a fluorine-based binder and a rubber-based binder.

The positive active material slurry according to an embodiment of the present invention may have a solid content of 30 to 80% by weight based on the total weight of the positive active material slurry.

The positive active material slurry according to an embodiment of the present invention may have a content of the positive active material of 55 to 99% by weight based on the total solid content of the positive active material slurry.

The positive electrode active material slurry according to an embodiment of the present invention may further include a conductive material.

In the positive electrode active material slurry according to an embodiment of the present invention, the conductive material may be selected from carbonaceous materials.

The present invention also provides a positive electrode including a positive electrode active material layer formed from the positive electrode active material slurry.

The present invention is a secondary battery including the positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, wherein the positive electrode has a positive electrode active material layer on one surface of the positive electrode current collector. A secondary battery having a thickness of 10 to 100 μm is provided.

The secondary battery according to the embodiment of the present invention may have an initial discharge capacity of 200 mAh / g or more and a capacity maintenance rate of 50% or more in 50 cycles.

The secondary battery according to the embodiment of the present invention may have a C-rate efficiency represented by the following formula 1 of 90% or more.

[Formula 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] × 100

According to the present invention, it has a low viscosity characteristic even at a high solid content concentration, can easily realize a high loading content with respect to the positive electrode active material, and exhibits excellent dispersibility when mixed with the positive electrode active material. A very stable phase can be maintained during period use or storage.

In addition, according to the present invention, since an organic solvent having a high environmental hazard (such as NMP) is not used as in the prior art, the positive electrode active material and the positive electrode including the same can be provided friendly to the environment.

Further, according to the present invention, a secondary battery having improved capacity characteristics and life characteristics can be provided. In particular, according to the present invention, not only a high initial discharge capacity but also a high discharge capacity after 50 cycles is exhibited, and a significantly improved capacity maintenance rate is exhibited.

Advantages and features of the present invention and methods for accomplishing them will become apparent with reference to the embodiments described in detail below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various different forms. However, this embodiment is provided in order to make the disclosure of the present invention complete, and to completely convey the scope of the invention to those having ordinary knowledge in the technical field to which the present invention belongs. Thus, the present invention is defined only by the claims. Hereinafter, a positive electrode active material coating solvent for a secondary battery according to the present invention, a positive electrode active material slurry containing the same, and a secondary battery manufactured therefrom will be described in detail.

The present invention provides a positive electrode active material coating solvent for a secondary battery containing N-ethylformamide. The coating solvent according to the present invention is characterized by exhibiting improved dispersibility when mixed with a positive electrode active material. Although the exact reason is not clear, it is expected to be due to the interaction between the N-ethylformamide according to the present invention and the positive electrode active material. The interaction is judged to be temporary or long-term influence of physical entanglement characteristics caused by hydrogen bond, covalent interaction including hydrophobic interaction, and non-covalent interaction. .

Such a phenomenon is a surprising synergistic effect due to the inclusion of N-ethylformamide, and is completely absent in formamide-based compounds (for example, formamide, N-methylformamide, etc.) having structural characteristics similar to those of N-ethylformamide. It has a big meaning in that it is not confirmed.

In addition, the positive electrode active material coating solvent of the secondary battery according to the present invention does not use a harmful organic solvent such as N-methyl-2-pyrrolidone (NMP), which is usually used, so that the positive electrode is more environmentally friendly. An active material layer and an electrode including the same can be provided.

The present invention also provides a positive electrode active material slurry containing N-ethylformamide.

Specifically, the positive electrode active material slurry may include a positive electrode active material, a binder, N-ethylformamide, and the like.

The positive electrode active material slurry according to an embodiment of the present invention may include a positive electrode active material represented by the following chemical formula 1.

[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂
(In the chemical formula 1, −0.5 ≦ x ≦ 0.6, 0 ≦ a, b, c ≦ 1, and x + a + b + c = 1.)

The positive electrode active material slurry according to an embodiment of the present invention can not only effectively suppress the capacity decrease of the positive electrode active material, but also the battery swelling due to the generation of gas due to the increase in the residual lithium content. In view of preventing such a problem, at least one selected from LiCoO ₂ , LiNiO ₂ , LiMnO _{2 and the} like may be included.

In addition, the positive electrode active material slurry according to an embodiment of the present invention may further include an additional positive electrode active material. For example, the additional positive electrode active material may be one or more selected from LiFePO ₄ , LiFeMnPO ₄ , LiFeMgPO ₄ , LiFeNiPO ₄ , LiFeAlPO ₄ , and LiFeCoNiMnPO ₄ , but is not limited thereto. It is not a thing.

In the positive electrode active material slurry according to an embodiment of the present invention, the binder is a component that assists the bonding of the metal oxide coating layer formed on the surface of the positive electrode active material or the bonding with the added conductive material. In addition, the substance is not limited as long as it is a substance usually used in the art, but specifically, it may be one or two or more selected from a fluorine-based binder and a rubber-based binder.

Examples of the fluorine-based binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), chlorotrifluoroethylene (CFTF), and polytetrafluoroethylene (PTFE). Is mentioned.

For example, examples of the rubber-based binder include styrene butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), and isoprene rubber (IR).

In the positive electrode active material slurry according to one embodiment of the present invention, the binder is an acrylic binder such as polyacrylonitrile, polymethyl methacrylate, polyacrylic acid; polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene, polypropylene, olefin binder; and One or more selected from cellulosic binders such as carboxymethylcellulose, hydroxypropylmethylcellulose, and regenerated cellulose may be further included.

The positive active material slurry according to an embodiment of the present invention may have a solid content of 30 to 80% by weight based on the total weight of the positive active material slurry. In this case, the balance is N-ethylformamide.

As described above, the positive electrode active material slurry according to the present invention uses N-ethylformamide as a dispersion medium, that is, a positive electrode active material coating solvent, thereby realizing a low viscosity despite a high solid content. As well as the dispersion stability can be remarkably enhanced.

The positive electrode active material slurry according to an embodiment of the present invention has a solid content of specifically 30 to 70% by weight, more specifically 40 to 60% by weight, based on the total weight of the positive electrode active material slurry. It may be.

Further, the positive electrode active material slurry according to one embodiment of the present invention may have a viscosity of 5,000 to 30,000 g / cm · s (measured at 23 ° C. using a Brookfield rotary viscometer at 20 rpm). Well, specifically, it may have a viscosity of 8,000 to 30,000 g / cm · s, more specifically 10,000 to 25,000 g / cm · s. The above-mentioned viscosity range corresponds to a low viscosity of 30 to 60% compared to the case where ordinary NMP or the like is used as a solvent.

For example, a positive electrode active material slurry having a solid content of 45% by weight (positive electrode active material: binder = 98: 2 (wt: wt)) may have a viscosity of 11,000 to 20,000 g / cm · s. it can. At this time, the rate of change in viscosity of the positive electrode active material slurry (viscosity after high temperature storage for 1 day / initial viscosity × 100, high temperature storage condition is 45 ° C.) has a range of 1 to 5%.

For example, a positive electrode active material slurry (positive electrode active material: binder = 98: 2 (wt: wt)) having a solid content of 50% by weight may have a viscosity of 15,000 to 23,000 g / cm · s. it can. At this time, the viscosity change rate of the positive electrode active material slurry (viscosity after high temperature storage for 1 day / initial viscosity × 100) has a range of 1 to 5%.

In the positive electrode active material slurry according to an embodiment of the present invention, the positive electrode active material may be included in an amount of 55 to 99% by weight based on the total solid content of the positive electrode active material slurry. It may be included at 99% by weight, more specifically 65-99% by weight. At this time, the balance is a binder.

Since the positive electrode active material slurry according to an embodiment of the present invention can form a stable dispersed phase even at a high solid content as described above, a change in physical properties (eg, viscosity) despite a relatively low viscosity. There is no fear of. Thus, the positive electrode active material slurry that forms a stable dispersed phase can maintain a very stable phase even during long-term use or storage, and can provide advantages in terms of process.

The positive electrode active material slurry according to an embodiment of the present invention may further include a conductive material. The conductive material is specifically a carbon-based material, which realizes the surface roughness of the positive electrode active material layer and provides conductivity. The conductive material is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, graphite such as natural graphite and artificial graphite; acetylene black, ketjen black, And carbon black such as channel black, furnace black, lamp black, and thermal black; and carbon fiber.

The conductive material may further include an additional conductive material in addition to the carbon-based material described above. For example, metallic materials such as copper, nickel, aluminum, silver, zinc and titanium; fibrous materials of the metals; polyaniline, polyacetylene, polypyrrole, polythiophenone, etc. Examples thereof include, but are not limited to, conductive polymers.

At this time, the usage capacity of the conductive material is not limited, but may be included at 0.01 to 10% by weight based on the total solid content of the positive electrode active material slurry. At this time, the content may be specifically 0.1 to 8% by weight, more specifically 0.5 to 5% by weight from the viewpoint of realizing improved capacity characteristics, but is not limited thereto. .

The present invention provides a positive electrode including a positive electrode active material layer formed from the positive electrode active material slurry described above.

A positive electrode according to an embodiment of the present invention includes a positive electrode active material layer formed with a uniform composition from the positive electrode active material slurry. In addition, the positive electrode active material layer may be formed with a high density. That is, the secondary battery employing the positive electrode including the positive electrode active material layer according to the present invention can remarkably improve the mobility of lithium ions in the electrolytic solution due to the improved ionic conductivity. Thereby, improved battery characteristics can be realized while reducing the amount of the additional conductive material.

The positive electrode according to an embodiment of the present invention can be manufactured by the following method.

Specifically, the method includes the steps of: adding a binder and a positive electrode active material to N-ethylformamide to produce a positive electrode active material slurry; and applying and drying the positive electrode active material slurry on a positive electrode current collector. Good.

The positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the secondary battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, It is possible to use baked carbon or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium or silver. At this time, the positive electrode current collector having a thickness of 3 to 500 μm can be used.

The application is not particularly limited, and can be performed by an ordinary known method in the art.

For example, the application may be performed by spraying or distributing the positive electrode active material slurry onto at least one surface of the positive electrode current collector, and then uniformly dispersing it using a doctor blade or the like.

For example, the application may be performed by die casting, comma coating, screen printing, or the like.

The drying is a process of forming a positive electrode active material layer and removing moisture remaining in the process. Under the present circumstances, the said drying is performed in the temperature range which is a grade from which a residual water | moisture content is removed, may be specifically performed at 50-200 degreeC, and may be specifically performed at 80-200 degreeC.

At this time, since the drying time is applied differently depending on the temperature, it cannot be uniformly determined, but it may be performed for 1 hour or more, preferably 5 to 24 hours, more preferably 5 to 12 hours. It may be.

The positive electrode active material layer manufactured by the above method may be formed to a thickness of 10 to 100 μm on at least one surface of the positive electrode current collector. Specifically, it may be formed with a thickness of 10 to 80 μm, more specifically 10 to 50 μm.

According to an embodiment of the present invention, since the loading variation is significantly reduced, it is possible not only to provide a positive electrode including a positive electrode active material layer with ensured uniformity in a very simple process, but also a high-density positive electrode. Providing an active material layer. Thereby, the secondary battery employing the positive electrode according to the present invention can remarkably improve the mobility of lithium ions in the electrolytic solution due to the improved ionic conductivity, and can exhibit excellent charge / discharge characteristics and life characteristics.

The present invention also provides a secondary battery including the positive electrode.

Specifically, the secondary battery includes a positive electrode according to an embodiment of the present invention, a negative electrode, and a separator between the positive electrode and the negative electrode, and the positive electrode is disposed on one surface of the positive electrode current collector. It is a lithium secondary battery in which an active material layer is formed with a thickness of 10 to 100 μm.

By adopting the positive electrode according to the present invention, the lithium secondary battery imparts improved lithium ion mobility in the electrolyte, and exhibits excellent charge / discharge characteristics and life characteristics.

The secondary battery according to an embodiment of the present invention may have an initial discharge capacity of 200 mAh / g or more. Specifically, the initial discharge capacity may be 200 to 300 mAh / g, more specifically 200 to 250 mAh / g.

In particular, a secondary battery according to an embodiment of the present invention, 50 cycles (50 ^th cycle) the change in discharge capacity even after use is minimized. Specifically, the secondary battery may have a capacity maintenance rate of 90% or more at 50 cycles.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more and a capacity maintenance rate of 50 to 90% in 50 cycles.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more and a capacity maintenance rate of 50 to 95% at 50 cycles.

In addition, the secondary battery according to the embodiment of the present invention may have a C-rate efficiency of 90% or more represented by the following formula 1.

[Formula 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] × 100

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more, a capacity maintenance ratio of 50 to 90%, and a C-rate efficiency of 90% to 99%.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more, a capacity maintenance rate of 50 to 95%, and a C-rate efficiency of 90% to 95%.

Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described.

The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator.

The positive electrode may include a positive electrode active material layer formed on at least one surface of a positive electrode current collector from the positive electrode active material slurry described above. Under the present circumstances, the lithium secondary battery by this invention can show the above-mentioned outstanding charging / discharging characteristics and lifetime characteristics by including the said positive electrode active material layer.

The negative electrode can be manufactured by applying a negative electrode active material slurry solution containing a negative electrode active material onto a negative electrode current collector and then drying. Under the present circumstances, the said negative electrode active material slurry solution may also contain the following components as needed. The negative electrode active material is not limited as long as it is a commonly used material. For example, carbon such as non-graphitizable carbon and graphite-based carbon; Li _x Fe ₂ O ₃ (0 ≦ x ≦ 1), Li _x WO ₂ (0 ≦ x ≦ 1), Sn _x Me _{1- x} Me ′ _y O _z (Me: Mn, Fe, Pb, Ge; Me ′: Al, B, P, Si, Groups 1 and 2 of the periodic table) Group 3 elements, halogens; metal composite oxides such as 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); lithium metals; lithium alloys; silicon alloys; tin alloys; SnO, SnO _{2, PbO, PbO 2, Pb} 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ metal oxides such as O _5; conductive polymers such as polyacetylene; Li-Co-N System metal composites; and the like. The negative electrode current collector is not particularly limited as long as it does not induce a chemical change in the secondary battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, Examples include titanium, calcined carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver or the like, and an aluminum-cadmium alloy. The negative electrode current collector, like the positive electrode current collector, may reinforce the binding force of the negative electrode active material by forming fine irregularities on the surface, film, sheet, foil, net, porous body, Needless to say, it may be formed in various forms such as foam and nonwoven fabric. At this time, a negative electrode current collector having a thickness of 3 to 500 μm can be used.

The separator is interposed between the positive electrode and the negative electrode, and an insulating thin film having high ion permeability and mechanical strength can be used. For example, the separator may have a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. Such a separator is not limited as long as it is a substance that is usually used, and examples thereof include chemical-resistant and hydrophobic olefin polymers such as polypropylene; sheets and nonwoven fabrics made of glass fiber or polyethylene; and the like. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as a separator.

The lithium secondary battery may further include an electrolytic solution.

The electrolyte solution may be a lithium salt-containing non-aqueous electrolyte solution. For example, the lithium salt-containing nonaqueous electrolytic solution may contain a nonaqueous organic solvent and a lithium salt. Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran (franc) 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1 , 3-Dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, propionic acid Aprotic organic solvents such as chill and the like. In this case, the non-aqueous organic solvent may be one or two or more mixed solvents selected from the above-mentioned aprotic organic solvents.

The lithium salt is not limited as long as it is a substance that is easily dissolved in the non-aqueous organic solvent. For _{example, LiCl, LiBr, LiI, LiClO} 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiAlCl 4, CH 3 SO 3 Li, CF 3 SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate and the like can be mentioned, and LiPF ₆ is most preferable.

The electrolyte solution may be an organic solid electrolyte. For example, one or more ionic dissociation selected from polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymer, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, etc. It may be a polymer containing groups.

The electrolyte solution may be an inorganic solid electrolyte. For _{example, Li 3 N, LiI, Li} 5 NI 2, Li 3 N-LiI-LiOH, LiSiO 4, LiSiO 4 -LiI-LiOH, Li 2 SiS 3, Li 4 SiO 4, Li 4 SiO 4 -LiI-LiOH, Examples include Li nitride such as Li ₃ PO ₄ -Li ₂ S-SiS ₂ , halides, sulfates, and the like.

The lithium secondary battery may further include an additional additive.

For example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes for the purpose of improving charge / discharge characteristics and flame retardancy , N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, and the like.

For example, for the purpose of imparting incombustibility, one or more selected from halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included.

For example, carbon dioxide gas may be further included for the purpose of improving high temperature storage characteristics.

The lithium secondary battery according to the present invention can be used not only for a battery cell used as a power source of a small device, but also preferably used as a unit battery for a medium-sized battery module including a large number of battery cells.

In addition, the battery module according to the present invention provides a battery pack that is included as a power source for a medium-sized device, and the medium-sized device includes an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug. An electric vehicle including a plug-in hybrid electric vehicle (PHEV), a power storage device, and the like can be provided, but is not limited thereto.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are for explaining the present invention more specifically, and the scope of the present invention is not limited by the following examples. It goes without saying that the following examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

Unless otherwise specified in the present invention, the unit of temperature is ° C., and unless otherwise defined, the unit of the amount used for each component is g.

(Evaluation method)
1. Evaluation of Charging / Discharging Capacity Using the secondary battery monocell (battery capacity 4.3 mAh) manufactured in the following examples and comparative examples, charging / discharging (0.5C) in a voltage interval of 3 to 4.5 V at 25 ° C. And discharged at 1C).

2. Evaluation of C-rate efficiency C-rate efficiency was measured in a high temperature environment (45 ° C.) using the secondary battery monocell (battery capacity 4.3 mAh) manufactured in the following examples and comparative examples. At this time, the C-rate efficiency is defined as the ratio of the capacity when a secondary battery charged at 0.5 C is discharged at 0.1 C and the capacity when discharged at 2 C as shown in the following formula 1. did.

[Formula 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] × 100

3. It was defined as the ratio of the discharge capacity after measured in Evaluation of the charging and discharging capacity of the life characteristics charging and discharging for 50 cycles (50 ^th discharging capacity) and the initial discharge capacity (1 ^st discharge capacity) (See Formula 2 reference ).

[Formula 2]
Capacity retention rate (%) = [(discharge capacity after 50 cycles) / (initial discharge capacity)] × 100

A surface-coated LiCoO ₂ cathode active material, a conductive material (carbon black), and a binder (polyvinylidene fluoride, PVdF) are mixed at a weight ratio of 95: 3: 2 to produce 20 g, and N-ethylformamide 20 g The positive electrode active material slurry (viscosity: 20,000 g / cm · s, viscosity change rate: 1%) was produced. The positive electrode active material slurry was applied to an aluminum (Al) thin film as a positive electrode current collector having a thickness of about 20 μm, dried at 130 ° C. for 2 hours, and then subjected to a roll press to produce a positive electrode. Lithium metal foil (foil) was used as the negative electrode. By adding LiPF ₆ to a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1: 2 as an electrolyte, a 1M LiPF ₆ non-aqueous electrolyte is obtained. Manufactured. A polymer cell type secondary battery monocell was manufactured by including a polyethylene separator (F2OBHE, thickness = 20 μm) between the positive electrode and the negative electrode and injecting the non-aqueous electrolyte. .

The initial charge / discharge capacity, C-rate efficiency, and life characteristics (capacity retention ratio) of the secondary battery monocell manufactured by the above method were evaluated and are shown in Table 1 below.

(Comparative Example 1)
In the production of the positive electrode active material slurry, positive electrode active material slurry (viscosity: 50,000 g / cm · s, viscosity change rate: N-methyl-2-pyrrolidone (NMP) instead of N-ethylformamide) is used as a solvent. 15%). Thereafter, a polymer cell type test secondary battery monocell was manufactured in the same manner as in Example 1.

The initial charge / discharge capacity, C-rate efficiency, and life characteristics (capacity retention ratio) of the secondary battery monocell manufactured by the above method were evaluated and are shown in Table 1 below.

The secondary battery according to the present invention not only showed a remarkable charge capacity, but also had an initial discharge capacity of 200 mAh / g or more and an excellent capacity maintenance rate.

Specifically, the secondary battery according to the present invention showed a high initial discharge capacity (203.9 mAh / g) and a C-rate efficiency of 91%. In particular, it was confirmed that the secondary battery according to the present invention has a capacity retention rate of 97%. Such a high capacity retention rate of the secondary battery according to the present invention is a remarkable effect of reaching 114% or more as compared with the comparative example.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings. Needless to say, this also belongs to the scope of the present invention.

Claims (12)

A solvent for coating a positive electrode active material of a secondary battery, comprising N-ethylformamide. A positive electrode active material slurry comprising a positive electrode active material, a binder, and N-ethylformamide. The positive electrode active material slurry according to claim 2, wherein the positive electrode active material is represented by the following chemical formula 1.
[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂
(In the chemical formula 1, −0.5 ≦ x ≦ 0.6, 0 ≦ a, b, c ≦ 1, and x + a + b + c = 1.) The positive electrode active material slurry according to claim 2 or 3, wherein the binder is selected from a fluorine-based binder and a rubber-based binder. The positive electrode active material slurry according to any one of claims 2 to 4, wherein the solid content is 30 to 80% by weight, based on the total weight of the positive electrode active material slurry. The positive electrode active material slurry according to any one of claims 2 to 5, wherein a content of the positive electrode active material is 55 to 99% by weight based on a total solid content of the positive electrode active material slurry. The positive electrode active material slurry according to any one of claims 2 to 6, further comprising a conductive material. The positive electrode active material slurry according to claim 7, wherein the conductive material is a carbon-based material. The positive electrode containing the positive electrode active material layer formed from the positive electrode active material slurry as described in any one of Claims 2 thru | or 8. A secondary battery comprising the positive electrode according to claim 9, a negative electrode, and a separator between the positive electrode and the negative electrode,
The positive electrode is a secondary battery in which a positive electrode active material layer is formed with a thickness of 10 to 100 μm on one surface of a positive electrode current collector. The secondary battery according to claim 10, wherein the initial discharge capacity is 200 mAh / g or more, and the capacity maintenance rate at 50 cycles is 90% or more. The secondary battery according to claim 10 or 11, wherein the C-rate efficiency represented by the following formula 1 is 90% or more.
[Formula 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] × 100